Power tools such as, for example, core drills, generally serve to drill holes into mineral materials such as for instance, concrete or bricks. In this context, the core drill normally has an electric motor, a transmission, a control unit or control device, a tool socket as well as a tool in the form of a core bit. Via the transmission, the electric motor drives the tool socket together with the tool. The transmission normally has two gears as well as a no-load state, which can be selected or actuated manually by the user of the core drill. By means of the individual gears, the motor speed is stepped up or down to the drive speed in different ways. In this manner, depending on the transmission ratio, either a high rotational speed and a low torque, or else a low rotational speed and a high torque can be available at the core bit. Once the core bit has been made to rotate, it cuts a circular hole into the material by means of its diamond-tipped cutting edge, thereby creating a cylindrical drill core. This drill core is removed from the drilled hole at the end of the drilling or cutting procedure.
A core drill according to the state of the art is disclosed in German patent application DE 10 2011 089 771. This power tool configured as a core drill comprises a drive means with a drive motor and a drive shaft, a driven means with a driven shaft, a transmission with a planetary gear train and an additional gear unit as well as a device for changing between a first and a second gear speed step of the planetary gear train.
A widespread problem encountered with the core drills according to the state of the art is to be seen in the unsynchronized transmissions, which makes changing from one gear into another gear awkward and time-consuming. Owing to the rotational speed difference between the motor (as the drive element) and the next gear (as the driven element) that is to be selected, changing gears with a unsynchronized transmission is only possible when the driven shaft is stationary (that is to say, not rotating) and the motor is turned off. In order to then finally change the gear, the driven shaft has to be turned by hand into the correct position until the corresponding gear wheels of the drive shaft and of the driven shaft are intermeshing and connected with a positive fit.